i965/fs: emit constants only once

i965/fs: emit constants only once

Before, we would lazily emit a MOV whenever we encountered a use of a
constant. Now that we have a dedicated file for SSA values, we can
instead only emit the MOV's once, which is more consistent and prevents
us from relying on CSE to re-combine the constants when they aren't
absorbed into the instruction.

total instructions in shared programs: 6078991 -> 6073118 (-0.10%)
instructions in affected programs:     402221 -> 396348 (-1.46%)
helped:                                1527
HURT:                                  0
GAINED:                                8
LOST:                                  2

v2: split this out from the previous commit (Jason)

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>